Trends of solar ultraviolet irradiance at Barrow, Alaska, and the effect of measurement uncertainties on trend detection

Abstract. Spectral ultraviolet (UV) irradiance has been observed near Barrow, Alaska (71° N, 157° W) between 1991 and 2011 with an SUV-100 spectroradiometer. The instrument was historically part of the US National Science Foundation's UV Monitoring Network and is now a component of NSF's Arctic Observing Network. From these measurements, trends in monthly average irradiance and their uncertainties were calculated. The analysis focuses on two quantities, the UV Index (which is affected by atmospheric ozone concentrations) and irradiance at 345 nm (which is virtually insensitive to ozone). Uncertainties of trend estimates depend on variations in the data due to (1) natural variability, (2) systematic and random errors of the measurements, and (3) uncertainties caused by gaps in the time series. Using radiative transfer model calculations, systematic errors of the measurements were detected and corrected. Different correction schemes were tested to quantify the sensitivity of the trend estimates on the treatment of systematic errors. Depending on the correction method, estimates of decadal trends changed between 1.5% and 2.9%. Uncertainties in the trend estimates caused by error sources (2) and (3) were set into relation with the overall uncertainty of the trend determinations. Results show that these error sources are only relevant for February, March, and April when natural variability is low due to high surface albedo. This method of addressing measurement uncertainties in time series analysis is also applicable to other geophysical parameters. Trend estimates varied between −14% and +5% per decade and were significant (95.45% confidence level) only for the month of October. Depending on the correction method, October trends varied between −11.4% and −13.7% for irradiance at 345 nm and between −11.7% and −14.1% for the UV Index. These large trends are consistent with trends in short-wave (0.3–3.0 μm) solar irradiance measured with pyranometers at NOAA's Barrow Observatory and can be explained by a change in snow cover over the observation period: analysis of pyranometer data indicates that the first day of fall when albedo becomes larger than 0.6 after snow fall, and remains above 0.6 for the rest of the winter, has advanced with a statistically significant trend of 13.6 ± 9.7 days per decade.